Methods for manufacturing printed circuit boards using a partial printing process and apparatus

ABSTRACT

An apparatus for preventing media deposition in predetermined locations comprising at least one blocking cap connected to a pressurized squeegee head is disclosed. In one embodiment, a method for partially printing a circuit board is disclosed. The method comprises inserting one or more blocking caps inside a pressurized squeegee head and printing with the pressurized squeegee head. In this way, defined areas on the surface of a stencil or screen, over which the blocking caps travel, are skipped. In another embodiment, a new method for manufacturing printed circuit boards is described, comprising placing a component on a circuit board, performing burn-in or testing of the component, partially printing attachment media on the circuit board with a pressurized squeegee head containing one or more blocking caps, and placing additional components on the circuit board. In one embodiment, the circuit board is sold or returned to the circuit board designer before performing the partial printing step.

[0001] This application is a divisional of U.S. patent application Ser.No. 09/434,944, filed Nov. 5, 1999, which is incorporated herein byreference.

FIELD

[0002] This invention relates generally to circuit board printingprocesses, and in particular, the present invention relates to methodsfor manufacturing printed circuit boards using a partial printingprocess and apparatus.

BACKGROUND

[0003] Surface mounting involves a planar mounting of components to acircuit board having a previously-applied conductive pattern. Thecomponents may be individual surface-mounted components, flip chips,tape automated bonding formats, and so forth. The conductive pattern formost components is obtained by a printing process that involvesdepositing a layer of attachment media, such as solder paste, onto astencil or screen. Solder is forced through openings in the stencil orscreen onto pads located on the circuit board below using a conventionalsqueegee blade. Alternatively, pressurized squeegee heads having twosqueegee blades can be used to apply the solder with the use of internalpressure, by either rolling it onto the stencil or forcing it throughthe stencil openings. This is in contrast to the conventional squeegeeblade in which solder paste is applied as it rolls in front of a singleblade. With either type of system, these processes produce a full orcomplete printing of the entire circuit board in one step, after whichthe components are placed on the circuit board in the usual manner.

[0004] Full printing, however, provides limited flexibility in terms ofbeing able to apply components at different times, as the surface overwhich the squeegee blade or head travels needs to be substantiallyplanar. Failure of a die after it and all of the other components havebeen added to the circuit board can be costly. As a result, manymanufacturers use various package processes, such as a controlledcollapse chip connection (C4) process, to produce a package containingthe die. In this way burn-in and testing of the die can take place priorto mounting it on the board. Such packages are costly to produce,however, and failure of the die after it is part of the package is alsoexpensive. Such cost concerns have caused many manufacturers to limittheir designs so as to produce smaller packages, eliminate large orheavy parts, avoid double-sided circuit boards, and so forth.

[0005] An alternative “two-step” printing process is sometimes used whendifferent pitches of print are desirable. The first step involves afirst printing or fluxing for the die. A second or “partial” printing isthen performed using a step-down stencil of varying thickness to blockthe pre-printed (or fluxed) die locations. Following the secondprinting, components (including surface mount components and the die)are placed on the circuit board in the usual manner. Typically, theentire process is performed in a clean room, although a clean room isonly necessary for placing the die on the circuit board. However, thesame problem of costly die failure exists.

[0006] Another means by which a “partial” print is obtained involvesdispensing solder paste through the needle of a syringe. This method isvery slow, however, because the paste is dispensed onto only one pad or“land” at a time. Clogging of needles is also a common problem with thismethod. Special dispense heads can be used to reduce the cycle time,although this increases the costs of the process significantly.Furthermore, due to the high cost of these processes, the complexity ofthe circuit board or module design is limited.

[0007] For the reasons stated above, there is a need in the art for amore convenient and economic method of manufacturing circuit boardsusing partial printing.

SUMMARY

[0008] An apparatus for preventing media deposition in predeterminedlocations comprising at least one blocking cap connected to apressurized squeegee head is disclosed. In one embodiment, a method forpartially printing a circuit board is disclosed. The method comprisesinserting one or more blocking caps inside a pressurized squeegee headand printing with the pressurized squeegee head. In this way, definedareas on the surface of a stencil or screen, over which the blockingcaps travel, are skipped.

[0009] In another embodiment, a new method for manufacturing printedcircuit boards is described, comprising placing a component on a circuitboard, performing testing of the component, partially printingattachment media on the circuit board with a pressurized squeegee headcontaining one or more blocking caps, and placing additional componentson the circuit board. In one embodiment, the circuit board is sold orreturned to the circuit board designer before performing the partialprinting step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view of two blocking caps secured inside apressurized squeegee head during a partial print operation in oneembodiment of the present invention.

[0011]FIG. 2 is an exploded front view of three blocking caps securedinside a pressurized squeegee head directly above previously-placed dicein one embodiment of the present invention.

[0012]FIG. 3 is a schematic illustration of a partially printed area ona circuit board in one embodiment of the present invention.

[0013]FIG. 4 is a perspective view of an alternative blocking cap in oneembodiment of the present invention.

[0014]FIG. 5A is a block diagram of a conventional package process inone embodiment of the present invention.

[0015]FIG. 5B is a block diagram of a conventional full printing endplacement process in one embodiment of the present invention.

[0016]FIG. 6 is a block diagram of a partial printing process in oneembodiment of the present invention.

DETAILED DESCRIPTION

[0017] An apparatus and method for partially printing circuit boards isdisclosed. In the following detailed description, reference is made tothe accompanying drawings which form a part hereof, and in which isshown by way of illustration specific embodiments in which theinventions may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that mechanical, procedural,electrical and other changes may be made without departing from thespirit and scope of the present inventions. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims,along with the full scope of equivalents to which such claims areentitled.

[0018] In one embodiment, a blocking cap capable of preventing anattachment media from being applied to a particular area is provided.Unlike conventional printing processes which spread attachment mediaacross the entire surface of a stencil or screen, the partial printingprocess of the present invention utilizes a blocking cap to create astrip where there is no printing in areas where previously-placedcomponents are located. In another embodiment, more than one blockingcap is used.

[0019] The blocking caps 110 fit inside a conditioning chamber 112 of apressurized squeegee head 114 as shown in FIG. 1. In another embodiment,the blocking caps 110 are integral with the pressurized squeegee head114, which is manufactured together with the blocking caps 110. Theblocking caps 110 allow defined areas 116 on the surface of a stencil(or screen) 118 to be skipped. The conditioning chamber 112 is formed bytwo angled plates 120, each of which is connected to an end plate 121.The conditioning chamber 112 is filled with an attachment media 122,such as solder paste. Squeegee blades 128, which are attached to theedges of each of the two angled plates 120 and to each of the two endplates 121 provide a seal to surrounding areas. In this way, attachmentmedia 122 is confined within the area of the conditioning chamber 112.

[0020] The attachment media 122 is deposited onto pads (not shown) on acircuit board array 126 located beneath the stencil 118. The attachmentmedia 122 enters the circuit board array 126 through apertures 124 inthe stencil 118. In the embodiment shown in FIG. 1, the blocking caps110 are secured to the conditioning chamber 112 with bolts 129. In analternative embodiment, rails are added to the inside of theconditioning chamber 112 so that the blocking caps 110 can be slid intoposition.

[0021] Although FIG. 1 shows two blocking caps 110, in practice, anysuitable number of blocking caps 110 can be used. The blocking caps 110can be any suitable size and shape as long as they prevent theattachment media 122 from being deposited onto the defined areas 116 andprovide adequate clearance along each side of previously-placedcomponents 130. Further, the blocking caps 110 should be deep enough toallow the pressurized squeegee head 114 to pass over the pre-placedcomponents 130 without making contact with the components 130. In theembodiment shown in FIG. 1, the blocking caps 110 are substantiallyrectangular-shaped objects, although the invention is not so limited. Inone embodiment, at least two of the vertical surfaces of the blockingcap 110, such as the sides of the blocking cap 110, are angled inwardlyat a suitable angle, such as about less than five (5) to about 45degrees. In another embodiment, the configuration of the pressurizedsqueegee head 114 and/or the conditioning chamber 112 is altered toaccommodate a particular shape or size of blocking cap 110, or aparticular number of blocking caps 110.

[0022] Each blocking cap 110 can be open on the bottom and have a lip132 along two sides of a bottom edge. The lips 132 help to prevent theattachment media 122 from leaking into the defined or blocked areas 116.In the embodiment shown in FIG. 1, neither end of the blocking cap 110has a lip, but is raised to provide clearance for the components 130that are located in the defined areas 116 (but secured to the boardarray 126). In another embodiment, the components 130 are about the sameheight as the stencil 118 and there is no need for the blocking caps 110to provide additional clearance above these components. In thisembodiment, the bottom edges of the ends of the blocking cap 110 aresubstantially flush with the bottom edges of the sides of the blockingcap 110, although any of the bottom edges can have lips 132 as needed.Any of a variety of components 130 may be secured to the board array 126prior to printing with the pressurized squeegee head 114, including, butnot limited to a bare die, a packaged component, and so forth.

[0023] In the embodiment shown in FIG. 1, each blocking cap 110 hasrounded edges and corners which may help to provide an even flow ofattachment media 122 around the exterior of the blocking cap 110. In oneembodiment, each blocking cap 110 is about 10 to 35 mm wide, about 10 to35 mm long, and about 10 to 35 mm tall. In a particular embodiment,there is about one (1) to four (4) mm clearance on either side of thecomponent 130, although this depends on the particular application.

[0024] As noted above, the blocking caps 110 are positioned over the topof a component 130 or a row of components 130, such as pre-built dieslocated on the board arrays 126. The components 130 stick up throughholes 131 on the surface of the stencil 118. The blocking caps 110 canbe aligned manually or automatically. In one embodiment, the blockingcaps 110 are aligned by hand over the components 130. In a particularembodiment, a measuring device is located along the side of theconditioning chamber 112 and a corresponding measuring device is locatedalong a frame on which the board array 126 is positioned. In this way,the blocking caps 110 can be accurately positioned over the components130 by placing them in the same location along the measuring device. Inan alternative embodiment, the blocking caps 110 are connected to aoptical alignment system, such as a machine vision system for properalignment.

[0025] The blocking caps 110 can be made from any suitable materialwhich is durable for the given operating conditions, does not decomposein the presence of the attachment media 122, and so forth. In oneembodiment, each blocking cap 110 is made from rubber, plastic,polyurethane or any suitable metal with a protective coating as is knownin the art. In another embodiment, the blocking caps 110 are made fromthe same material as any type of squeegee blade known in the art,including metal blades.

[0026] The blocking caps 110 can be used in any application in which aconventional pressurized squeegee head 114 and attachment media 122 areused. Typically, a thixotropic media such as solder paste (solder) isused as an attachment media in surface mount technology (SMT), althoughin certain applications it may be desirable to use nonthixatropicmedias. In one embodiment, liquid flux or adhesive paste is used insteadof solder paste. In one exemplary embodiment, solder paste is usedtogether with a DEK ProFlow print head from DEK Printing Machines Ltd.,in Weymouth, England. In another embodiment a MPM Rheometric Pump PrintHead from MPM, Inc., in Franklin, Mass. is used.

[0027]FIG. 2 is an exploded front view of three blocking caps 110secured inside a pressurized squeegee head 114. In this embodiment, theboard array 126 contains three rows of pre-built dice 230. The pre-builtdice 230 have been secured to the board array 126 in any suitablemanner. Placement of the blocking caps 110 as shown prevents depositionof the attachment media 122 onto the row of dice 230 and onto an areaimmediately surrounding the dice 230.

[0028] Any conventional type of stencil 118 can be used. In oneembodiment, the stencil 118 is about the same height as the component130 which has been previously placed on the board array 126. In theembodiment shown in FIG. 2, the stencil 118 has openings 232 in thelocations of the previously placed dice 230. The openings can extendthrough to the top of the stencil 118, but the invention is not solimited.

[0029] Referring again to FIG. 1, in operation, the pressurized squeegeehead 114 travels along the surface of the stencil 118 in eitherdirection as shown. As the head 114 travels, the attachment media 122 isforced out of the open bottom of the conditioning chamber 112. As aresult, the attachment media 122 is spread along the surface of thestencil 118, except in the defined areas 116 blocked by the blockingcaps 110. In this way, a “partial” print is obtained. Once printing iscompleted, the stencil 118 itself is relatively “clean” as there istypically little, if any, attachment media 122 remaining on the surfaceof the stencil 118.

[0030]FIG. 3 shows a board 326 under an alternative stencil 318. In thisembodiment, the alternative stencil 318 is larger than the underlyingboard 326. Again, the path of travel of the blocking caps 110 is shownas defined areas 116, which has no print due to the presence of theblocking caps 110. The printed area path 316 is shown between thedefined areas 116. The alternative board 326, also has board definedareas 320. Again, the defined areas 116 correspond with the location ofthe previously placed components 130 plus some additional clearance oneither side of these components 130. The alternative stencil 318 hasnumerous apertures 324 through which the attachment media 122 is spreadto the board 326 below. Again, there are also holes 131 large enough fortheir respective components 130 to pass through.

[0031]FIG. 4 shows an alternative blocking cap 410 which has squareedges and corners rather than rounded edges. In this embodiment,flexible seals 412 are added to the lower outer edges of the alternativeblocking cap 410 to provide adding protection against unwanted solderdeposition, although the invention is not so limited. The flexible seals412 can be made from any suitable material, such as metal or plastic.Further testing is necessary to determine the suitably of any particularblocking cap design for a given application and the desirability ofadding one or more seals to the previously described blocking caps (110)shown in FIG. 1. In an alternative embodiment, the flexible seals 412are connected directly to the pressurized squeegee head (not shown inFIG. 4). Refinements to the system of the present invention can be madeby testing a variety of blocking caps on pressurized squeegee heads andadjusting the shape, size and position as necessary.

[0032] In another embodiment, a new method of manufacturing printedcircuit boards is described. This method allows the process of printingto be separated from other steps in the process of manufacturing circuitboards. Specifically, after an individual part is placed on the circuitboard, it can be sold to a different manufacturer or returned forfinishing to the original company which designed the circuit board. In aconventional process, it is not practical to print on a circuit board,once a part has been placed on the circuit board. By using the blockingcap, it is now feasible and practical to print on a circuit board afterone or more parts have been placed on it. Since the blocking cap blocksthe flow of the attachment media, one can selectively choose where mediashould be placed.

[0033] One example of a prior art process is shown in FIG. 5A. This canbe any type of ball grid array (BGA) package process, such as thecontrolled collapse chip connection, i.e., C4 package process, wellknown in the art. The C4 package process is also referred to as the“flip chip” process. In this process, solder bumps are prepared on thedice and/or on a substrate in step 510. A wafer sort is performed instep 512 to eliminate bad dice. The dice are sawed and a package iscreated in step 514. Creating of the package typically involves placingeach die on a substrate and attaching an array of solder balls to thebottom surface of the substrate, as is known in the art. Testing, whichcan include burn-in and/or other types of testing known in the art, isperformed on the C4 package in step 516.

[0034] A prior art printing and placement process is shown in FIG. 5B.In this process, a full printing of a bare circuit board is performed instep 518. This includes printing with a conventional squeegee orpressurized print head for surface mount components as well as printingfor placement of a package, such as a C4 package. Surface mountcomponents are placed on the circuit board in step 520 and the C4package is placed on the circuit board in step 522. This is followed byconventional reflow in step 524 and testing in step 526, such as anin-circuit test (ICT). Other required and alternative steps known in theart for the C4 package process and the printing and placement processare not discussed herein.

[0035]FIG. 6 shows one type of novel process which is made possible withuse of the blocking caps of the present invention. A bare circuit boardis printed or fluxed for a bare die using any suitable method in step610, with the bare die being placed on the bare circuit board in step612, although the invention is not so limited. Any type of component canbe added to the bare circuit board in step 612, including apreviously-developed package, if desired. However, since it is notnecessary to use packages when using the novel partial printing processof the present invention, considerable cost-savings and flexibility canbe obtained if packages are not used. A burn-in at an elevatedtemperature for a suitable length of time and/or testing of the die isperformed in step 614. At this point, the circuit board containing thebare die can be shipped to another location, if desired, as shown instep 616, with the buyer performing the remaining steps in the process.

[0036] Alternately, the process can continue in the same location. Instep 618, a partial print is made on the circuit board containing thepreviously-added die, using a pressurized squeegee head containing oneor more blocking caps as described above. The remaining parts are placedon the circuit board in step 620. This is followed by conventionalreflow in step 622 and testing, such as the ICT, in step 624. It shouldbe understood that other types of soldering, such as wave soldering canbe performed in step 622. Other tests which are known in the art canalso be performed in step 624. Further, in some processes it may benecessary to perform other steps such as a curing step, repair step, andso forth. Other variations are also possible depending on the type ofSMT process being used, whether a double-sided circuit board is beingproduced, etc.

[0037] In one embodiment, the blocking cap is used with the conventionalmobile module process. In this way, the mobile module process can bealtered so that the bare die is added and tested prior to adding all ofthe other components.

[0038] The system of the present invention is more economical andefficient than conventional partial printing processes. By combining apressurized squeegee head with one or more novel blocking caps, it isnow possible to attach a component to a bare circuit board before theSMT process is started, instead of creating a package and then placingit on the circuit board. This feature provides previously unattainableflexibility in the manufacturing process since one or more costlycomponents can be added, burned-in, and/or tested prior to printing andplacing all the remaining components. As a result, the process ofprinting can now be easily separated from other steps in the process ofmanufacturing circuit boards. Furthermore, by eliminating use of apackage, the overall height of the completed circuit board is reduced.Limiting the height of the completed circuit board in this way isextremely important, particularly in applications where overall space islimited, such as in notebook computers.

[0039] Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for manufacturing printed circuit boardscomprising: placing a component on a circuit board; partially printingattachment media on the circuit board with a pressurized squeegee headcontaining at least one blocking cap, wherein no attachment media isprinted onto the component; and placing additional components on thecircuit board.
 2. The method of claim 1 wherein the component is a die.3. The method of claim 2 further comprising testing the die prior to thepartial printing step.
 4. The method of claim 1 wherein the attachmentmedia is solder paste, liquid flux or adhesive paste.
 5. The method ofclaim 1 further comprising shipping the circuit board to anotherlocation before the partial printing step.
 6. The method of claim 1further comprising reflowing the attachment media and performingin-circuit testing.
 7. The method of claim 1 wherein the pressurizedsqueegee head contains two blocking caps.
 8. The method of claim 1wherein the pressurized squeegee head contains three blocking caps. 9.The method of claim 1 wherein the pressurized squeegee head containsfour or more blocking caps.
 10. A method comprising: placing a componenton a circuit board; shipping the circuit board to another location;partially printing attachment media on the circuit board with apressurized squeegee head containing at least one blocking cap, whereinno attachment media is printed onto the component; and placingadditional components on the circuit board.
 11. The method of claim 10wherein the component is a die.
 12. The method of claim 11 furthercomprising testing the die prior to the partial printing step.
 13. Themethod of claim 10 wherein the attachment media is solder paste, liquidflux or adhesive paste.
 14. The method of claim 10 further comprisingreflowing the attachment media.
 15. The method of claim 14 furthercomprising performing in-circuit testing.
 16. A method for manufacturingprinted circuit boards comprising: placing a die on a circuit board;partially printing solder paste on the circuit board with a pressurizedsqueegee head containing at least one blocking cap, wherein no solderpaste is printed onto the die; and placing additional components on thecircuit board.
 17. The method of claim 16 further comprising testing thedie prior to the partial printing step.
 18. The method of claim 17further comprising shipping the circuit board to another location beforethe partial printing step.
 19. The method of claim 18 further comprisingreflowing the solder paste.
 20. The method of claim 19 furthercomprising performing in-circuit testing.